Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit of a fine pattern on e.g. a Si substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices is being accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (wavelength: 193 nm) is employed, about 45 nm is presumed to be the limit. Under the circumstances, as an exposure technique for the next generation employing an exposure wavelength shorter than 45 nm, EUV lithography is expected to be prospective, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser. In this specification, EUV light means light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive indices of substances at such a wavelength are close to 1, whereby it is not possible to use a conventional refractive optical system like photolithography employing visible light or ultraviolet light. Therefore, in EUV lithography, a reflective optical system, i.e. a reflective photomask and mirror, is employed.
A mask blank is a stacking structure before being subjected to pattering, to be used for the production of a photomask. In the case of an EUV mask blank, it has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, are formed in this order on a substrate made of e.g. glass. As the reflective layer, it is common to use a multilayer reflective film having a high refractive index layer and a low refractive index layer alternately stacked to have the light reflectance improved when the layer surface is irradiated with EUV light. For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing Ta or Cr as the main component, is used.
On the absorber layer of the EUV mask blank, a low reflective layer to a mask pattern inspection light is usually formed. In order to detect the presence or absence of a defect of pattern after the formation of a mask pattern, a light ray which has the wavelength region of deep ultraviolet light (190 to 260 nm) is employed. In the pattern inspection employing the light ray having the above-mentioned wavelength region, presence or absence of the defect of pattern is examined by the difference of reflectance between an area where the low reflective layer and the absorber layer have been removed by a patterning process and an area where the low reflective layer and the absorber layer remain, namely, the contrast of reflected light at the surfaces of these areas. In order to increase the sensitivity of the detection of mask pattern, the contrast is required to be increased. For this purpose, it is usually required that the low reflective layer has a low reflectance at the above-mentioned wavelength region, namely, the reflectance at the wavelength region is 15% or less.
Patent Document 1 discloses that it is preferred to form a low reflective layer comprising an oxide of tantalum-boron alloy (TaBO) or an oxynitride of tantalum-boron alloy (TaBNO) on an absorber layer comprising a nitride of tantalum-boron alloy (TaBN) since the reflectance at the wavelength region (190 nm to 260 nm) of mask pattern inspection light is thereby low.
Patent Document 2 and 3 disclose that it is preferred to form a low reflective layer comprising a metal, silicon (Si), oxygen (O) and nitrogen (N) on an absorber layer in order to adjust the reflectance at the wavelength region (190 nm to 260 nm) of mask pattern inspection light.
In any of Patent Document 1 to Patent Document 3, a layer composed of an oxide or a layer composed of an oxynitride is used as the low reflective layer. This is for the purpose of adding oxygen to the low reflective layer to improve the low reflective function at the wavelength in the vicinity of from 190 to 260 nm. On the other hand, however, when a layer composed of an oxide or a layer composed of oxynitride is employed as the low reflective layer, there is a problem of a decrease in etching rate as described below.
At the time of production of a mask for EUVL, a dry etching process is employed usually when a pattern is formed in an absorber layer and a low reflective layer, and as the etching gas, a chlorine type gas (or a mixed gas containing a chlorine type gas) (herein after collectively referred to as a chlorine type gas) or a fluorine gas (or a mixed gas containing a fluorine type gas) (hereinafter collectively referred to as a fluorine type gas) is usually used. When a film containing Ru or a Ru compound is formed as a protective layer on a reflective layer for the purpose of preventing damage to the reflective layer by etching process, a chlorine type gas is mainly used as the etching gas for the absorber layer since the damage to the protective layer is small. On the other hand, in a case where a layer composed of an oxide or a layer composed of a oxynitride is employed as the low reflective layer, when a chlorine type gas is used, the etching rate is lower than when a fluorine gas is used. Thus, a fluorine gas is commonly used in the etching process of a low reflective layer.
In order to form a pattern in an absorber layer and a low reflective layer, it is usually required to carry out two-step etching process, i.e. to carry out an etching process using a fluorine type gas for the low reflective layer and to carry out an etching process using a chlorine type gas for the absorber layer. However, when such a two-step etching process is carried out, two etching chambers are required, and thus the process becomes complicated and there is concern about contamination during transfer between chambers. Further, in a case where two steps of the etching process are carried out in a chamber, different types of gases i.e. a fluorine type gas and a chlorine type gas are mixed, and thus problems may occur such as contamination of the chamber and destabilization of the process.
The present inventors conducted extensive studies to solve such problems and previously found that when the low reflective layer is a film (SiN film) containing Si and N, it has low reflective layer properties in the entire wavelength region (190 to 260 nm) of an inspection light for a mask pattern, and an improvement in the etching rate in an etching process employing a chlorine type gas is possible. An EUV mask blank based on this knowledge is disclosed in Patent Document 4.
However, the EUV mask blank disclosed in Patent Document 4 has a low reflective layer which is a film containing Si, and thus there is a concern that a problem about adhesion with a resist for a mask pattern as described in paragraph [0003] of Patent Document 5 may occur. Patent Document 5 includes a description about adhesion between a Si-containing film and a resist which is applied at the time of forming a pattern, and it discloses that in the case of a Si-containing film, the adhesion with a resist is insufficient. That is, in the case where the low reflective layer is a Si-containing film, the adhesion with a resist which is applied at the time of forming a mask pattern is insufficient, and when a fine resist pattern is formed, specifically, when a fine resist pattern having a width of about 100 nm or less, problems may occur such that a resist pattern disappears or a resist pattern falls down and becomes a defect of the resist pattern.